American U.S. Pat. No. 4,911,381 discloses such a seat underframe. That document is incorporated with reference to the present description, with only the characteristics that are specific to the invention being described in detail.
Such known seat underframes offer the advantage of procuring improved behavior in the event that they are subjected to very large amounts of deceleration, e.g. due to a crash. The energy absorption device connects the top of the front leg to the vertex of the triangular base so as to maintain the relative positions of the legs under conditions of normal use, while permitting relative motion generating energy absorption in the event of a violent impact.
Nevertheless, in such known seat underframes, it is not easy to adapt the value of the trip threshold, above which the energy absorber device is tripped, as a function of the use that is to be made of the underframe (weight of the seat to be supported, number of seats, position of the underframe relative to the floor of the aircraft and to the other seats, etc.).
The energy absorption devices that are in general use have standard characteristics which cannot be modified in order to choose the quantity of energy that can be absorbed from a wide range of values and with accuracy.
In addition, an underframe made in that way is heavy and requires a large number of parts that must be manufactured and assembled. More particularly, if the characteristics of the energy absorption device are to be modified, the following must be modified accordingly: the dimensions, the relative positions, and the various constituent parts of the underframe. The anchor points, the longitudinal dimensions and the overall size of the energy absorption device are generally modified as a function of the value of the energy to be absorbed. In this way, it is necessary to design a specific underframe as a function of almost each energy absorption device used.